Electronic security systems have been developed for a number of years. Recently, electronic technology has been used in combination with a traditional door lock to provide smart look functionality. A disadvantage of such smart lock resides in that replacement of an existing lock is inevitable, and the installation process of the new smart lock is relatively complicated and time-consuming. Moreover, installation of the smart lock also requires a specialized manual labor to use tools for mounting the smart lock and a corresponding system.
Accordingly, there is a smart lock that may be attached to the existing lock. One of the key elements of the smart lock is a gear mechanism to be driven by a motor for rotating a thumb knob of the existing lock. The gear mechanism should have high efficiency and increased torque capability to save energy. However, the relatively high torque, i.e., the rotational resistance of the motor when the motor is not operating, makes it difficult to turn the smart lock manually.